1. Field of the Invention
The present invention relates to methods for manufacturing micromachined devices and in particular methods for manufacturing structural silicon germanium layers having contact regions with low electrical resistivity and low strain gradient suspended regions suitable for the formation of micromachined devices.
2. Description of the Related Technology
In the past years, the need for integrating Micro Electro Mechanical Systems (MEMS) (or also referred to as micromachined devices) with prefabricated Complementary Metal Oxide Semiconductor (CMOS) electronics to produce compact and improved devices has set some limitations on the processing temperature of the active MEMS material. Post-processing monolithic integration requires reduced maximum processing temperatures in the range of, for example, 420 to 520° C.
Polycrystalline silicon germanium is an attractive material for MEMS post-processing, as it allows good electrical, mechanical and thermal properties at temperatures that are lower than the temperatures required for polycrystalline silicon processing.
However, emerging interest in fabricating MEMSs on temperature sensitive substrates, such as passive and flexible substrates, requires further reduction of the processing temperature of the active or structural layers. Using substrate materials such as benzocyclobutene (BCB), silicone, polymide (PI) or polyethylene terephthalate (PET) may limit the maximum processing temperature to, for example, 300° C. or lower.
There is thus a continuous need for further reduction of the processing temperature of polycrystalline silicon germanium structural layers.
In EP 1 801 067 A2, a method for manufacturing structural silicon germanium layers for surface machined MEMS devices at temperatures substantially below 400° C. is disclosed. The method comprises deposition of amorphous silicon germanium (a-SiGe) at a temperature below 400 degrees Celsius using plasma enhanced chemical vapor deposition (PECVD). However, the as-deposited a-SiGe has degraded electrical and mechanical properties, including high stress, strain gradient and electrical resistivity, all of which are not acceptable for functional and reliable MEMS structural layers. The method further comprises a laser annealing step with restricted laser fluences for improving the mechanical properties (stress and/or strain) of the as-deposited a-SiGe. By using the restricted laser fluences, a low strain gradient can be achieved. However, the structural layers thus obtained have high sheet resistance. It is suggested that by further tuning of the parameters of the as-deposited a-SiGe (for example increasing the germanium (Ge) content) and by using a higher number of laser pulses a lower resistitivity may be achieved. However a high Ge content is not fully compatible with standard CMOS processing. Moreover a high Ge content may affect device reliability.
Thus, there remains a need for further reduction of the processing temperature of polycrystalline silicon germanium structural layers without degradation to the electrical and mechanical properties.